The present invention relates to a vehicle detector and classifier.
There is a growing world-wide market for systems for detecting and classifying road vehicles. Road tolling, road pricing, and traffic monitoring and control are becoming increasingly important. Such systems are also likely to be of use in the automated or intelligent highways of the future. Accurate, low cost, low maintenance sensors are required which can not only detect but classify vehicles for automatic tolling and priority lane enforcement. The invention is also applicable to aircraft ground control and military vehicle classification.
One form of vehicle detector in common use comprises one or two large loops of electrically conductive material which are arranged on or in a road, substantially in the plane of the road surface. Vehicles are detected by the reduction in the inductance of the loop caused by the metallic vehicle body passing thereover.
Whilst detectors of this kind can be used to classify vehicles according to their length, they do not detect the axles or wheels of the vehicle and hence classification according to the number, type and position of axles or wheels is not possible. Such classification is, however, the accepted and sensible way to classify vehicle types.
Axle classification can be achieved by using a pneumatic tube or piezoelectric sensor in addition to the inductive loop. However, this adds to the cost, is impractical on unsurfaced roads, has a limited life span and cannot detect individual wheel configurations.
It is therefore highly desirable to provide an inductive loop vehicle detector which can detect vehicle wheels.
EP-A-0,649,553 describes a vehicle detector comprising at least one and up to eight inductive loops, having a width (extending in the direction of travel) only substantially equal to the bearing surface on the ground of the vehicle wheel (i.e. about 0.3 m for heavy goods vehicles or 0.15 m for light vehicles). The or each loop is arranged substantially in the plane of the road surface. This arrangement is able to detect vehicle wheels although the influences of the metallic masses of the body and of-the tyres of the vehicle on such,small loops are opposed.
The reason given in EP-A-0,649,553 for these opposite influences is that the loop or loops constitute a first electrical circuit, and the metallic mass of the vehicle causes a variation in the magnetic field produced by the first circuit, which in turn causes a variation in the flux linking a second circuit formed by the metallic masses in the wheel and, more particularly, by the torus formed by the wheel rim and the metallic tyre reinforcements, thus inducing a current in the second circuit.
We believe that such reasoning is erroneous since it would cause a change in the inductance of the loop opposite to the results actually described and shown in EP-A-0,649,553. In fact, whilst the large conducting area of a vehicle body causes a decrease in the loop inductance due to eddy currents, the vehicle tyre contains ferrous metal but in the form of steel bands or webbing, not in the form of a large conducting sheet. The vehicle tyre thus has a high magnetic permeability, but a relatively low electrical conductivity, and causes an increase in the loop inductance.